1. Field of the Invention
The present invention relates to an amplifier circuit which allows a wide potential range of input signals.
2. Description of the Background Art
FIG. 11 is a block diagram showing positioning control of a magnetic head of an HDD (hard disk drive) device. For moving the magnetic head to a track specified by a desired value T, a signal processing circuit SP drives a motor driver MD through an operational amplifier OP and a filter FL. The motor driver MD actuates a motor M. Consequently, a sensor SN senses the position of the moved magnetic head to apply the position information to the signal processing circuit SP through a position detecting circuit PD. The signal processing circuit SP applies a motor control signal MS to the operational amplifier OP in full consideration of the desired value T and the position information.
When the position of the magnetic head deviates largely from the desired value, a large motor control signal MS is applied to the operational amplifier OP. Being used as a voltage follower, the operational amplifier OP, if having an insufficient dynamic range of the input signal potential range, cannot drive the motor driver MD suitably.
Thus desirable is a large dynamic range of the input signal potential range of the operational amplifier OP.
FIG. 12 is a circuit diagram of an operational amplifier 2000 that is an example of conventional amplifier circuits. The operational amplifier 2000 processes input signals given from a non-inverting input terminal 1 and an inverting input terminal 2 to apply an output signal to an output terminal 3.
The potential range of the input signal allowed to be inputted to the input terminals 1 and 2 of the operational amplifier 2000 is limited not only by a potential V.sub.cc at a potential point 4 and a potential GND (=0) at a potential point 5 but also by a saturation voltage V.sub.sat of a transistor 302 and a base-emitter voltage V.sub.BE of transistors 101 and 102.
The upper limit of the input signal potential range is limited to {V.sub.cc -(V.sub.sat +V.sub.BE)}.
The operational amplifier as an example of the conventional amplifier circuits presents the problem that its input signal potential range is limited by the saturation voltage and base-emitter voltage of the transistors forming the operational amplifier.
There has recently been a tendency toward the requirement of a low power supply voltage V.sub.cc, while the saturation voltage V.sub.sat and base-emitter voltage V.sub.BE of the transistors are not greatly reduced. Therefore the dynamic range of the input signal potential range deteriorates as the power supply voltage V.sub.cc decreases.